Re: Renaissance class U.S.S. Aries
In TNG, it seems to pretty much be the backbone of the fleet. I would imagine the registry issue is because Starfleet kept building Excelsiors. Why replace a perfectly reliable design? Indeed, as with all ship classes, no two have the exact same internals, so as you keep building them, they stay up to date with technology and building methods, and the Excelsior seems vaunted and reliable, so no need for a replacement for quite a while.
STAR TREK: 1964-1965½, 1966-1969, Jan. 21-23 1972, 1979-2001, 2003-2005, 2009-?